The present invention relates to a method and an apparatus for manufacturing batteries, and particularly to a method and an apparatus for welding current collector plates to electrode plate groups.
For the structure of rechargeable batteries such as nickel metal hydride rechargeable batteries, a prismatic structure has been proposed, wherein an electrode plate group 21 as shown in FIG. 3A is accommodated together with a liquid electrolyte within a prismatic case (not shown) having a rectangular cross-section and an open top end which is closed with a lid member (not shown). Such structure serves to increase the electrode surface area as much as possible within a restricted space, and in addition to raising the reactive efficiency of the electrode plates, thereby enabling a large current to be taken. It therefore significantly affects battery characteristics.
More specifically, as shown in FIG. 3B in detail, the electrode plate group 21 is constituted by alternately superimposing a plurality of positive electrode plates 22 made of foamed nickel and a plurality of negative electrode plates 23 formed of punched metal with an active material of hydrogen-absorption alloy powder in paste form, each of the positive electrode plates 22 being respectively covered with bag-shaped separators 24 made of polypropylene non-woven cloth having openings on one side, so that the positive and negative electrode plates 22, 23 are layered upon one another with intervening separators 24 therebetween.
The plurality of positive electrode plates 22 and the plurality of negative electrode plates 23 respectively have their lead portions 22a, 23a on one side, that are protruded outwards on the opposite sides. The positive electrode lead portions 22a are formed by compressing the foamed nickel and seam-welding lead plates on one side thereof by ultrasonic welding, while the negative electrode lead portions 23a are constructed of part of the electrode plates which is left uncoated with the active material. A positive electrode current collector 25 and a negative electrode current collector 26, both made of either a nickel sheet or a nickel-plated steel sheet, are abutted perpendicularly to the side edges of the lead portions 22a, 23a on the opposite sides of the electrode plate group 21, and joined thereto by welding.
Welding methods using electronic beams have been proposed for the joining of the collector plates 25, 26 to the lead portions 22a, 23a. One such example is illustrated in FIG. 4 and FIGS. 5A and 5B. As shown, a plurality of corrugations 27 (seven in the illustrated example) are formed in the collector plates 25, 26 preliminarily at certain spaces in the longitudinal direction, and solder material 28 such as nickel solder is applied within the elongated indentations of the corrugations 27 on the side contacting the lead portions 22a, 23a. Under a state wherein these collector plates 25, 26 are tightly pressed against the lead portions 22a, 23a, electronic beams 30 are irradiated on the backside of the collector plates 25, 26 opposite from the side contacting the lead portions 22a, 23a in a vacuum atmosphere. The electronic beams 30 are scanned in the direction in which the electrode plates are layered as indicated by the arrow, whereupon the collector plates 25, 26 are heated and the solder material 28 melts. The collector plates 25, 26 are thus welded to the side edges of the lead portions 22a, 23a. This welding action is performed to the plurality of locations in the lengthwise direction of the collector plates simultaneously or successively. The lead portions 22a, 23a are formed with a pair of position locating holes 29a through which corresponding position locating pins 29 are passed whereby the side edges of the lead portions 22a, 23a are aligned to form flat planes, so that the welding of the collector plates 25, 26 can be favorably performed.
The production equipment for manufacturing such electrode plate groups includes a processing chamber capable of vacuum exhaustion, in which electronic beam irradiating means are arranged. Electrode plate groups 21, to which collector plates 25, 26 have been assembled, are carried into this processing chamber, and when a vacuum is drawn to a desired degree, the welding operation is performed. Thereafter, the pressure within the processing chamber is returned to an atmospheric level, whereupon the processed electrode plate group 21 is removed, and next electrode plate group is carried in.
There can also be an arrangement, wherein a preliminary chamber and a post-processing chamber are respectively arranged adjacent the processing chamber, the former being evacuated to the vacuum level of the processing chamber during the welding of the preceding electrode plate group, and the latter being arranged to return the pressure therein from the vacuum to the atmospheric level.
Japanese Laid-open Patent Application No. 53-114748 discloses an electronic beam welding apparatus applicable to mass-production of one type of works, wherein a plurality of chambers having an identical constitution are arranged in line for cyclic processes. An automatic attachment/removal mechanism is provided for successively connecting a single electronic beam irradiating means to each of the chambers corresponding to the welding process in each chamber.
The problem faced by the method wherein electrode plate groups are carried into a single processing chamber and the processing chamber is then vacuumed is that a considerably long time is required until a vacuum of desired level is drawn. Even if the vacuuming speed is increased, because of the water which is much contained within the positive electrode plates 22 and which is released gradually as time passes, the evacuation needs to be carried on for a long time until a vacuum of more than a predetermined level is created. Thus the production efficiency is extremely poor, since it took time until the welding can actually be started after the electrode plate group has been introduced into the processing chamber. Also, the cost is considerably high, and so this method was hardly applicable to mass-production.
If a multiplicity of processing chambers were provided and welding performed at the same time in these chambers, mass-production could be possible. However, such equipment with a large number of processing chambers would be horrendously expensive, hence impracticable.
In terms of time consumed for creating a high vacuum, it is also the case with a system wherein a preliminary chamber and a post-processing chamber are arranged adjacent the processing chamber, since the production tact time in such system is known to be as long as about 300 sec. In order to realize mass-production, the tact time must be reduced to about 50 sec.
The apparatus disclosed in the above-mentioned Japanese Laid-open Patent Application No. 53-114748 provides no solution to the problem that it takes time for drawing a vacuum when handling a work which contains much water such as the electrode plate group. The system requires a plurality of expensive chambers capable of drawing a high vacuum, and a complex structure for switchably connecting the electronic beam irradiating means to each of the chamber, hence the equipment cost is extremely high.
Next, another problem encountered in prior arts is described referring to FIG. 10A, which is a schematic illustration of a prior art welding method. Normally, when welding the collector plates and the electrode plate groups together by scanning an electronic beam in a direction in which the electrode plates are layered, the signal waveform for the scanning of the electronic beam is triangular, in order to make the scanning speed constant so as to apply heat uniformly along the scanning direction.
However, when applying heat to a collector plate 122 by scanning the electronic beam 123 along the widthwise direction of the collector plate, both side edges of the irradiated portion 124 receives less heat than a middle part because there is no heat applied on their outer sides and heat is more readily dissipated. As a result, there occurred a problem of incomplete welding because part of the collector plate 122 was left unmelted. If the output or time for irradiation of electronic beam was increased to augment the entire amount of heat, the heat applied to the middle part of the collector plate 122 would become excessive and it would only lead to another problem that separators interposed between electrode plates are damaged, which causes short-circuiting between the electrode plates.
In view of these problems, additional scannings 126, 127 are usually performed once or a plurality of times in spots or over a very short distance at both side edges of the collector plate 122 in addition to the overall scanning 125 over the entire width of the collector plate 122, as shown in FIGS. 10A and 10B. According to such method, however, since the additional scannings 126, 127 are required at both side edges, which take 35 ms and 25 ms respectively, in addition to the scanning 125 of the entire width of the collector plate 122 which takes 90 ms, as illustrated in FIG. 10B, the scanning time necessary for welding one line sums up to 210 ms. If a collector plate 122 is to be joined to an electrode plate group 121 at 5 locations, it takes 1050 ms. Thus the conventional welding process is very time-consuming, which is partly the cause of low productivity in the manufacture of electrode plate groups for batteries.
In view of the problems encountered in the prior arts, it is an object of the present invention to enhance the efficiency in the process steps for welding current collector plates to electrode plate groups and to reduce the production tact time without increasing cost for the equipment, so as to realize mass-production of electrode plate groups for batteries.
To accomplish the above object, the present invention provides a battery manufacturing method comprising the steps of:
drying a plurality of positive electrode plates and a plurality of negative electrode plates;
layering the positive electrode plates and negative electrode plates alternately upon one another with intervening separators therebetween, thereby constituting electrode plate groups;
assembling collector plates to the electrode plate groups;
transferring the electrode plate groups with the collector plates from one to another of a plurality of vacuum chambers having different vacuum levels; and
performing a welding operation using electronic beams in one of the plurality of vacuum chambers in which a vacuum of sufficient level to effect irradiation of electronic beams is created. Since the electrode plates are first dried in the drying step, when the electrode plate groups are introduced into the vacuum chambers, the level of vacuum within these chambers can be increased in a very short period of time. Therefore, the overall production tact time can be reduced in proportion to the number of vacuum chambers. The method of the present invention is thus applicable to mass-production of batteries at low cost.
More specifically, the plurality of vacuum chambers are arranged adjacent each other such that the vacuum level is gradually increased from an upstream side to a downstream side as the electrode plate groups with the collector plates are transferred from one to another of the vacuum chambers, and the welding operation is performed in one vacuum chamber in which the vacuum level is highest. Thereby, there is very little variance in the vacuum levels between adjacent vacuum chambers, and each chamber is capable of creating a predetermined vacuum therein in a short tact time, respectively. Therefore, the vacuum of a level necessary for performing welding operation can be created during the time in which the electrode plate groups are transferred from one to another of the plurality of vacuum chambers. Moreover, the expensive electronic beam irradiating device is provided only in one vacuum chamber, and so the production equipment can be constructed at low cost.
The transferring of the electrode plate groups with the collector plates from one vacuum chamber to an adjacent vacuum chamber can be accomplished in an efficient manner by performing a series of operations wherein: a first gate provided on the upstream side of a first vacuum chamber is opened; the electrode plate groups with the collector plates are introduced into the first vacuum chamber; the first gate is closed; the first vacuum chamber is vacuumed to a predetermined level; a second gate provided on the downstream side of the first vacuum chamber is opened; and the electrode plate groups with the collector plates are moved into an adjacent second vacuum chamber.
After the welding operation has been performed, the electrode plate groups with the collector plates are introduced into a post-processing vacuum chamber before being moved to the outside. Therefore, the processed electrode plate groups can be moved to the outside without reducing the vacuum level in the vacuum chamber in which the welding operation has been performed. Since this post-processing vacuum chamber receives only the processed electrode plate groups, and since the chamber is vacuumed before the electrode plate groups are transferred thereinto, a desired level of vacuum can be obtained in a very short period of time. Therefore, it is only necessary to provide one such post-processing chamber, and so it gives practically no effects on the production tact time.
When opening a gate of a vacuum chamber on the most upstream side and when opening a gate of the post-processing vacuum chamber, a dry gas is introduced into these chambers. Thereby, outer air containing water therein can hardly enter into the vacuum chambers, and the time until a desired level of vacuum is reached can accordingly be reduced.
In the step of performing a welding operation using electronic beams, an electronic beam is irradiated on the collector plate and scanned along a direction in which the electrode plates are layered, using a signal waveform which causes the scanning speed at either end of the collector plate to become slower than the scanning speed in a middle portion of the collector plate. Since the irradiation time of electronic beam is prolonged at either end of the collector plate, the amount of heat applied to the collector plate at opposite ends is increased, whereby the entire width of the collector plate can be welded to the electrode plate group uniformly. Also, because no additional scanning is necessary at either end of the collector plate, the welding can be performed in a very short period of time, and the productivity is accordingly enhanced.
The signal waveform is obtained by combining a triangular wave and a sine wave at predetermined ratios, and specifically, the heat balance between the middle part and either end of the collector plate can be optimized if the signal waveform has triangular wave to sine wave ratios in the range of 1 to 4:1.
In order to implement the above battery manufacturing method, the present invention provides a welding apparatus for joining the current collector plates to electrode plate groups comprising:
a plurality of preliminary vacuum chambers having different vacuum levels therein arranged adjacent each other such that the vacuum level is gradually increased from an upstream side to a downstream side;
a processing chamber having a highest level of vacuum therein arranged adjacent one of said plurality of preliminary vacuum chambers, in which an electronic beam irradiating device is arranged so that a welding operation is performed therein;
a post-processing chamber connected to the processing chamber, in which the electrode plate group, to which the current collector plates have been joined, is introduced before being taken out to the outside; and
means for transferring in succession the electrode plate group with the collector plates from a preliminary vacuum chamber having a lowest vacuum level to the post-processing chamber, through the plurality of another preliminary vacuum chambers having higher vacuum levels and the processing chamber. Thereby, the production tact time is reduced while maintaining the equipment cost low, and mass-production of batteries at low cost can be realized as described in the foregoing.
More specifically, preliminary vacuum chambers located on the upstream side and having lower vacuum levels are provided with vacuum means including a mechanical booster pump, whereas preliminary vacuum chambers located on the downstream side and having higher vacuum levels are provided with a vacuum means including a composite turbo molecular pump, and the post-processing chamber is provided with a vacuum means including both of the mechanical booster pump and the composite turbo molecular pump. The mechanical booster pump which exhibits high performances for creating a low degree of vacuum of about 10 Pa and a composite turbo molecular pump which exhibits high performances for creating a higher level of vacuum are combined suitably to constitute respective vacuum means taking advantage of their respective characteristics, so that expensive composite turbo molecular pumps are used in a fewer number, thereby cutting equipment cost, while ensuring that a desired level of vacuum is efficiently created.
As mentioned above, the preliminary vacuum chamber located on the most upstream side and the post-processing chamber are provided with means for introducing therein one of dry air and an inactive gas, so that outer air containing water can hardly enter into these chambers, and the time until a desired level of vacuum is reached is accordingly reduced.
While novel features of the invention are set forth in the preceding, the invention, both as to organization and content, can be further understood and appreciated, along with other objects and features thereof, from the following detailed description and examples when taken in conjunction with the attached drawings.